Optical fiber gratings are well established components that have been used in industry for communication and sensing applications. These devices are formed by exposing a germanosilicate fiber to spatially periodic intensity from a high-power ultraviolet (UV) source. Interaction with the UV light affects molecular binding within the optical fiber core and creates refractive index perturbations. The perturbations form a grating structure. Based on periodicity, the grating will selectively reflect or couple light at a specific band of wavelengths to guided or nonguided modes of the waveguide.
Long period gratings typically have a plurality of index perturbations of width w spaced apart by a periodic distance .LAMBDA., where typically 50 .mu.m .ltoreq..LAMBDA..ltoreq.1500 .mu.m. The perturbations are formed within the core of the waveguide and form an angle with the longitudinal axis of the waveguide. The waveguide is designed to transmit broadband light at a wavelength centered about .lambda.. The spacing of the perturbations is chosen to couple transmitted light in the region of at least one wavelength from the guided mode into lossy non-guided modes in the cladding thereby reducing in intensity the band of light centered about a plurality of coupling wavelengths.
Vengsarkar et al. (U.S. Pat. No. 5,641,956) describe an optical waveguide sensor arrangement which comprises an optical waveguide having guided modes, lossy non-guided modes, and a long period grating coupling the guided modes to the lossy non-guided modes. The light contained in the non-guided modes interact with surface defects on the optical waveguide and is rapidly attenuated. These modes are referred to as lossy. The long period grating can be used as a sensor because it converts light traveling in the guided modes of the optical waveguide to the lossy non-guided modes of the optical waveguide at one or more wavelengths as determined by various environmental parameters being measured. This produces a wavelength transmission spectrum functionally dependent on the parameter sensed.
Long period gratings are typically used to couple light from the core mode to the forward propagating cladding mode to result in a transmitted signal. However, if long period gratings are to be used in reflection, external methods must be implemented to return the signal through the input fiber. One way this is achieved is by placing a reflector or filter over the entire fiber endface after the long period grating. In so doing, a reflective metal such as gold or a dielectric is deposited onto the fiber endface surface. Since the reflector covers the entire fiber endface, the cladding modes coupled by the long period grating are reflected as well as the fundamental mode. Placement of the mirrored fiber endface with respect to the long period grating is critical so the cladding modes are properly extinguished upon their reflection from the fiber endface and prior to returning back through the long period grating. Failure to extinguish the cladding mode light before re-entry into the long period grating results in recoupling the light in the cladding modes back into the fundamental mode. One way to avoid this is to place or provide a long segment of lower index buffered fiber after the long period grating to absorb light in the cladding mode. Hence, only light in the fundamental mode will be transmitted to the end of the fiber where it will be reflected back through the input fiber. The problem with this design is that the device becomes unnecessarily large and the packaging requirements become more stringent.
A similar method involves placing a reflector only within or on the fiber core. This is done by writing additional broadband reflective gratings after the long period grating. Because only the core signal is reflected, most of the cladding mode is essentially stripped. Thus, no significant recoupling of the cladding mode occurs. However, this technique requires expensive and time consuming fabrication processes.
Alternatively, a long period grating may be written with partial coupling of light into the cladding mode. Based on coupling conditions and the recoupling of reflected light back into the fiber, partial coupling to the cladding will be maintained and a long period grating signal with limited isolation results. This result is useful for some applications but numerous applications require greater coupling efficiency. Specific examples include chemical and biological sensing applications where the long period grating is brought into contact with various solutions. The solutions disturb the boundary conditions for the long period grating. If the isolation of the long period grating is small, the signal from the long period grating will be lost. Therefore, large isolation levels are required.
An object of the present invention is to provide a long period grating based single-ended optical device which produces high isolation of the long period grating in reflection and good coupling efficiency.
Another object of the present invention is to provide a long period grating based single-ended optical device which has large isolation levels.
Another object of the present invention is to provide a long period grating based single-ended optical device whose reflecting element is not sensitive to external perturbations.
Another object of the present invention is to provide a long period grating based single-ended optical device which is compact and rugged.